vbox: requires installation of yasm
vbox: fix dependencies of generated files

Express that the target binary files depend on the generated files not
the source files. The old expression seems to confuse Make, which then
logs a bogus error like the following

      COMPILE  Runtime/common/err/errmsg.o
  genode-x86-g++: error: Runtime/common/err/errmsg.cpp: No such file or directory
  genode-x86-g++: fatal error: no input files
  compilation terminated.
  make[2]: *** [Runtime/common/err/errmsg.o] Error 1
  make[1]: *** [virtualbox-runtime.lib] Error 2